Add impending deprecation notice to README.

Closes #9

README.md

@@ -10,6 +10,18 @@
 A require hook for loading single-file vue components in node. Useful for
 testing without having to spin up web browsers!
 
+## Impending Deprecation Notice
+
+[The Vue team is going to be creating official testing tools][vue-testing] based
+on [`avoriaz`] which already has complete examples of how to test Vue components
+in node-based testing frameworks without spinning up web browsers. My suggestion
+is for users of this library to migrate to [`avoriaz`] and then to the official
+testing tools once those are available. Once the official testing tools are
+available, I will mark this package as deprecated on npm.
+
+Usage of this library will also require changes to your `vue-loader`
+configuration starting with version `^12.0.0`. See issue #9 for more details.
+
 ## Usage Example
 
 Here is an example of using `vue-node` with [AVA]. The process should be similar